Propylene is the second most important feedstock in the petrochemical industry, after ethylene. It is the raw material for a wide variety of products, including polypropylene, which accounts for nearly two-thirds of all demand. In 2008, worldwide sales of propylene reached a value of over 90 billion U.S. dollars, and demand continues to increase.
There are two traditional routes to propylene production: steam cracking, whereby naphtha or other hydrocarbons are reacted with steam to make light olefins; and fluid catalytic cracking (FCC), which is the refinery operation that breaks down larger hydrocarbons to produce naphtha-weight components for gasoline, as well as olefins and heating oils.
Propane dehydrogenation (PDH) can also be used to produce propylene. Metathesis of ethylene and butane is yet another route to propylene production.
There are currently two commercial processes to produce propylene from methanol: the methanol-to-olefin (MTO) process, which produces roughly 50% ethylene and 50% propylene, and the methanol-to-propylene (MTP) process, which produces 100% propylene. PDH and MTO/MTP are “on demand” processes that are cost-effective when oil prices are high and prices of other alternative energy sources such as coal or natural gas are low.
Methanol-to-propylene (MTP) conversion is an emerging technology that is starting to be commercialized in some areas of the world where feedstocks for conventional processes are in short supply. In the MTP process, methanol is dehydrated to produce dimethyl ether, which is then converted to propylene, with byproducts such as C2, C4, C5, and C6 olefins, aromatics, and paraffins. After passing through a downstream separation train that usually includes multiple distillation columns, many of these byproducts are recycled to the main MTP reactor to increase propylene production. For example, the condensed overhead from the de-ethanizer column, which typically contains 90 wt % C2, is sent back to the reactor.
The non-condensed portion of the de-ethanizer overhead contains inerts such as hydrogen and CO that must be purged from the process. However, the overhead stream also contains valuable C1 and C2 hydrocarbons, which are lost from the process in the purge stream, which is typically used as fuel.
FIG. 1A is a detailed schematic for a standard MTP conversion process. The process shown in FIG. 1A involves ten principal pieces of equipment (along with various compressors, heat exchangers, separators, etc.), as follows (from left to right on the figure):                Methanol recovery column, 125;        Dimethyl ether (DME) reactor, 102;        Methanol-to-propylene (MTP) reactor, 104;        Quench column, 106;        Debutanizer column, 112;        DME removal system first column, 119;        DME removal system second column, 120;        Dehexanizer column, 114;        De-ethanizer column, 127; and        Propane-propylene splitter column, 129.        
According to the figure, fresh methanol, 101, from an outside source is routed as part of feed stream, 150, to DME reactor, 102. DME stream, 142, emanating from DME reactor 102, is then split into two streams: Stream, 103, which passes through heat exchange steps to MTP reactor, 104, to better control the reactor temperature; and stream, 143, which mixes with other recycle streams and is then sent to the reactor 104.
Resulting stream, 105, from MTP reactor 104 is passed to quench column, 106. Resulting stream, 107, from quench column 106 is compressed in compressor, 108. The resulting compressed stream, 109, is separated in separator, 146, into a liquid stream, 110, containing mostly C3-C5 hydrocarbons, and a vapor stream, 111, containing mostly, C1-C4 hydrocarbons.
Heavier hydrocarbon-containing liquid stream 110 is sent to debutanizer, 112, and separated into a liquid stream, 113, containing mostly C4-C5 and heavier hydrocarbons, and a vapor stream, 117, containing mostly C3-C4 and lighter hydrocarbons.
Heavier hydrocarbon-containing liquid stream 113 is sent to dehexanizer column, 114, and separated into a liquid stream, 115, containing mostly C5+ hydrocarbons, and a vapor stream, 116, containing mostly C4-C5 hydrocarbons. Liquid stream 115 can be sent for use in gasoline.
Lower hydrocarbon-containing stream 111 and stream 117 from debutanizer 112 are sent for treatment in DME removal system 118, which includes two columns, 119 and 120. Streams 111 and 117 enter first column 119. Stream, 121, containing mostly C1-C3 hydrocarbons, is withdrawn from first column 119 and sent to second column, 120, where it is contacted with methanol, 122, and water, 123. Liquid stream, 124, containing mostly methanol, DME, and water, is withdrawn from the bottom of second column 120. Stream 124 is sent back to the process at position C on the schematic, where it enters the methanol recovery column, 125.
Vapor stream, 126, containing mostly C1-C3 hydrocarbons, is withdrawn from the top of second column 120 and sent to de-ethanizer 127. Liquid stream, 128, containing mostly C3 hydrocarbons, is sent to propane-propylene splitter column, 129. Propylene in vapor form is withdrawn from the top of propylene/propane splitter column 129 and then condensed (condenser not shown) to produce liquid propylene product, 130. Liquid propane, 131, is withdrawn from the bottom of column 129.
Returning to de-ethanizer column 127: A vapor stream, 132, containing mostly C1 and C2, as well as some inerts (typically H2 and CO) is withdrawn from the top of column 127, then compressed in compressor, 133. The resulting compressed stream, 151, is routed to separator, 147. The non-condensed portion, 134, of compressed stream 151, containing mostly inerts and some residual C1 and C2 hydrocarbons, is withdrawn as a purge stream, which can be sent for use as fuel gas, 135.
The condensed portion, 136, of compressed stream 151, containing mostly C1 and C2 hydrocarbons, is split into two portions, one of which is recycled to the de-ethanizer column. The other portion of stream 136 joins stream 116 (containing mostly C4-C5 hydrocarbons) and stream 140 (discussed below) and is sent as hydrocarbon recycle stream, 141, back to the process at position D on the schematic. Hydrocarbon recycle stream 141 joins stream 143, which is then routed to MTP reactor 104.
Returning to first DME column 119: A liquid stream, 137, containing mostly C4 hydrocarbons, is withdrawn from the bottom of column 119. A portion, 138, of this stream is sent with propane stream 131 to make liquid petroleum gas (LPG), 139.
The remaining portion, 140, of stream 137 joins streams 116 and 136 and is sent as hydrocarbon recycle stream, 141, back to the process at position D on the schematic. The recycle stream 141 contains mostly C2-C5 hydrocarbons. As discussed above, this stream joins stream 143 and is routed to MTP reactor 104.
Returning to quench column 106: The bottoms stream from this column is split into two streams: Stream, 144, is heat-exchanged and joins streams 141 and 143 to be routed to MTP reactor 104; stream 145 joins stream 124 (both of which contain mostly methanol, DME, and water), to be routed to methanol recovery column 125.
The bottoms stream from methanol recovery column 125—which contains mostly water—is split into two streams: Stream 123, is routed to the second column 120 of DME removal system 118 at position B; stream 148 is purged.
Stream, 149, from the top of methanol recovery column 125, contains mostly methanol and DME, and joins stream 101 to be muted as feed stream 150 to DME reactor 102.
FIG. 1B is a greatly simplified schematic for the MTP conversion process shown in FIG. 1A. Referring to the figure, feed stream 150 is routed to reactor train, 160. Referring back to FIG. 1A, reactor train 160 consists of dimethyl ether (DME) reactor, 102; MTP reactor, 104; quench column, 106; methanol recovery column, 125; and associated equipment.
Heavier hydrocarbon-containing liquid stream 110 is sent to debutanizer 112 and separated into liquid stream 113, containing mostly C4-C5 and heavier hydrocarbons, and vapor stream 117, containing mostly C3-C4 and lighter hydrocarbons.
Heavier hydrocarbon-containing liquid stream 113 is sent to dehexanizer 114 and separated into liquid stream 115, containing mostly C5+ hydrocarbons, and vapor stream 116, containing mostly C4-C5 hydrocarbons. As discussed previously, liquid stream 115 can be sent for use in gasoline.
After passing through the DME removal system, 161 (columns 119 and 120 in FIG. 1), stream 117 is sent as vapor stream 126, containing mostly C1-C3 hydrocarbons, to de-ethanizer 127. Liquid stream 128, containing mostly C3 hydrocarbons, is sent to propylene/propane splitter column 129. Propylene in vapor form is withdrawn from the top of propylene/propane splitter column 129 and condensed (condenser not shown) to produce liquid propylene product 130. Liquid propane 131 is withdrawn from the bottom of column 129.
Vapor stream 132, containing mostly C1 and C2, as well as some inerts, is withdrawn from the top of column 127, then compressed and condensed (compressor and condenser not shown in FIG. 1B). The non-condensed portion, 134, contains mostly inerts and some residual C1 and C2 hydrocarbons and is withdrawn as a purge stream, which can be sent for use as fuel gas, as discussed above.
The condensed portion, 136, containing mostly C1 and C2 hydrocarbons, is split into two portions, 136a and 136b. Portion 136a is recycled to the de-ethanizer column. Referring back to FIG. 1A, portion 136b joins streams 116 and 140 and is sent as hydrocarbon recycle stream 141 back to the process at position D on the schematic. Hydrocarbon recycle stream 141 joins stream 143, which is then routed to MTP reactor 104.